The present invention relates to improvements in two layer optical elements. In particular, the present invention is direct to providing an two-layer diffractive optical element which can be used in heads-up or head mounted display.
Typical holographic heads-up and head-mounted displays and head-up displays have relied on the use of Bragg selectivity for the combining function. The combiner combines light from a more-or-less completely monochromatic source such as a video display screen with ambient light made up of the fill spectrum. The frequency of the monochromatic light and the grating period of the holographic optical element are used to determine the characteristics of the combiner, including its placement and orientation, so that the monochromatic light approaches the combiner at the so-called "Bragg angle" to maximize the reflection of the monochromatic light. Deviations from the prescribed placement can lead to deviations in the angle and reduce the reflected light, thus reducing the ability of the element to act as a combiner.
Bragg selectivity requires the use of thick holograms Thick holograms are generally fabricated by exposing film with thick emulsions to laser beam interference patterns. They require tedious, precise alignment conditions between the laser beams, and controlled environmental conditions for exposure and subsequent development. Thus, high quality thick holograms are typically very expensive.
Thin holographic optical elements, on the other hand, are often referred to as diffractive optical elements (DOEs). DOEs can be fabricated with photolithographic techniques (such as are disclosed in U.S. Pat. Nos. 4,895,790; 5,161,059 and 5,218,471 to Swanson et al., the entire disclosures of which are hereby incorporated by reference) in a manner similar to the fabrication of integrated circuits and very compatible with mass production. Furthermore, since thin DOE's (unlike thick HOEs) can be represented entirely by a two-dimensional surface profile, they can be replicated very inexpensively through injection molding or embossing.
Due to the relatively high cost of thick HOE's, most prior art combiners for head-mounted displays have not used any HOEs or DOEs. In such cases, most typically the combiners are made with little or no magnifying power. Typically, magnification using refractive and/or reflective optics requires the use of curved optical elements. The use of curved optics for combiners results in larger aberrations of the transmitted image and often results in larger, bulkier optical systems. In order to reduce aberrations, a flat combiner is often used in conjunction with a curved optical element. This can reduce the bulkiness of the optical system and can reduce the aberrations, but at the expense of increased reflections, which decrease the system transmission efficiency and generate additional ghost images. These problems become even more severe as the field of view of the head-mounted or heads-up display increases.
With the advent of multimedia virtual reality displays, there is a need in the art for a low-cost, lightweight optical element for incorporation into a head-mounted display to be used as a combiner and for which power can be incorporated without introducing aberrations or requiring the use of additional optical components. Similarly, for head-up displays, there is a need for a low-cost power combiner that does not introduce aberrations in the scene viewed by, for example, an automobile driver. In both cases freedom to deviate from the necessity of locating the combiner at the Bragg angle is also very helpful.